Towards More Functional Java using Generators and Filters

Last time we saw how to use lambdas as predicates, and specifically how to use them with the Java 8 Collection#removeIf method in order to remove elements from a map based on the predicate. In this article we will use a predicate to filter elements from a stream, and combine it with a generator to find the first open port in a specific range. The use case is a (micro)service-based environment where each new service binds to the first open port it finds in a specific port range. For example, suppose we need to limit the port range of each service to the dynamic and/or private ports (49152 to 65535, as defined by IANA). Basically we want to choose a port at random in the dynamic port range and bind to that port if it is open, otherwise repeat the process until we find an open port or we have tried more than a pre-defined number of times.

The original pre-Java 8 code to accomplish this looked like the following:

There are a few things to note here. First, the method returns an Integer to indicate that it could not find an open port (as opposed to throwing an exception, which might or might not be better). Second, there are two mutable variables assignedPort and count, which are used to store the open port (if found) and to monitor the number of attempts made, respectively. Third, the while loop executes so long as as the maximum number of attempts has not been exceeded. Fourth, a conditional inside the loop uses a port checker object to determine port availability, breaking out of the loop if an open port is found. Finally, a ternary expression is used to check the assignedPort variable and return either null or the open port.

Taking a step back, all this code really does is loop until an open port is found, or until the maximum attempts has been exceeded. It then returns null (if no open port was found) or the open port as an Integer. There are two mutable variables, a loop, a conditional inside the loop with an early break, and another conditional (via the ternary) to determine the return value. I'm sure there are a few ways this code could be improved without using Java 8 streams. For example, we could simply return the open port from the conditional inside the loop and return null if we exit the loop without finding an open port, thereby eliminating the assignedPort variable. Even so it still contains a loop with a conditional and an early exit condition. And some people really dislike early returns and only want to see one return statement at the end of a method (I don't generally have a problem with early exits from methods, so long as the method is relatively short). Not to mention returning null when a port is not found forces a null check on callers; if a developer isn't paying attention or this isn't documented, perhaps they omit the null check causing a NullPointerException somewhere downstream.

Refactoring this to use the Java 8 stream API can be done relatively simply. In order to accomplish this we want to do the following, starting with generating a sequence of random ports. For each randomly generated port, filter on open ports and return the first open port we find. If no open ports are found after limiting our attempts to a pre-determined maximum, we want to return something that clearly indicates no open port was found, i.e. that the open port is empty. I chose the terminology here very specifically, to correspond to both general functional programming concepts as well as to the Java 8 API methods we can use.

Without any explanation you can probably read the above code and tell generally what it does, because we are declaring what should happen, as opposed to listing the explicit instructions for how to do it. But let's dive in and look at the specifics anyway. The refactored method returns an OptionalInt to indicate the presence or absence of a value; OptionalInt is just the version of the Optional class specialized for primitive integers. This better matches the semantics we'd like, which is to clearly indicate to a caller that there may or may not be a value present. Next, we are using the generate method to create an infinite sequence of random values, using the specified IntSupplier (which is a specialization of Supplier for primitive integers). Suppliers do exactly what they say they do - supply a value, and in this case a random integer in a specific range. Note the supplier is defined using a lambda expression.

The infinite sequence is truncated (limited) using the limit method, which turns it into a finite sequence. The final two pieces are the filter and findFirst methods. The filter method uses a method reference to the isAvailable method on the portChecker object, which is just a shortcut for a lambda expression when the method accepts a single value that is the lambda argument. Finally, we use findFirst which is described by the Javadocs as a "short-circuiting terminal operation" which simply means it terminates a stream, and that as soon as its condition is met, it "short circuits" and terminates. The short-circuiting behavior is basically the same as the break statement in the original pre-Java 8 code.

So now we have a more functional version that finds free ports with no mutable variables and a more semantically correct return type. As we've seen in several of the previous articles in this ad-hoc series, we are seeing common patterns (i.e. map, filter, collect/reduce) recurring in a slightly different form. Instead of a map operation to transform an existing stream, we are generating a stream from scratch, limiting to a finite number of attempts, filtering the items we want to accept, and then using a short-circuiting terminal operation to return the value found, or an empty value as an OptionalInt.

As you can probably tell, I am biased toward the functional version for various reasons such as the declarative nature of the code, no explicit looping or variable mutation, and so on. In this case I think the more functional version is much more readable (though I am 100% sure there will be people who vehemently disagree, and that's OK). In addition, because we are using what are effectively building blocks (generators, map, filter, reduce/collect, etc.) we can much more easily make something generic to find the first thing that satisifies a filtering condition given a supplier and limit. For example:

Now we have a re-usable method that can accept any generator and any predicate. For example, suppose you want to find the first random number over two billion if it occurs within 10 attempts, or else default to 42 (naturally). Assuming you have a random number generator object rand, then you could call the findFirst method like this, making use of the orElse method on Optional to provide a default value:

So as I mentioned in the last article on predicates, there is a separation of concerns achieved by using the functional style that simply is not possible using traditional control structures such as the while loop and explicit if conditional as in the first example of this article. (*) Essentially, the functional style is composable using basic building blocks, which is another huge win. Because of this composability, in general you tend to write less code, and the code that you do write tends to be more focused on the business logic you are actually trying to perform. And when you do see the same pattern repeated several times, it is much easier to extract the commonality using the functional style building blocks as we did to create the generic findFirst method in the last example. To paraphrase Yoda, once you start down the path to the functional side, forever will it dominate your destiny. Unlike the dark side of the Force, however, the functional side is much better and nicer. Until next time, arrivederci.

You can find all the sample code used in this blog and the others in this series on my GitHub in the java8-blog-code repository.

(*) Yes, you can simulate functional programming using anonymous inner classes prior to Java 8, or you can use a library like Guava and use its functional programming idioms. In general this tends to be verbose and you end up with more complicated and awkward-looking code. As the Guava team explains:

Excessive use of Guava's functional programming idioms can lead to
verbose, confusing, unreadable, and inefficient code. These are by
far the most easily (and most commonly) abused parts of Guava, and
when you go to preposterous lengths to make your code "a one-liner,"
the Guava team weeps.